How To Megasquirt Your 2nd Gen RX-7

Introduction

The Megasquirt has become a very popular ECU for the RX-7. It is an extremely capable and inexpensive DIY ECU. However as the Megamanual says, "Experimental means that you are responsible for sorting out some details of your fuel injection that are specific to your application. Do-it-yourself means that you save money by assembling it yourself with a few basic tools and comprehensive instructions.". This means that out of the box, some versions of the Megasquirt require modification to run the rotary. In fact, some versions of the Megasquirt require one or two modifications for almost every application but as the rotary has a unique ignition system, it can be a bit more involved then most installations.

Notice I said "some versions of the Megasquirt". We'll get back to exactly what that means shortly.

Don't let this deter you, however. Behind the Megasquirt is a huge community of users that are more then willing to help out. Indeed, as the Megasquirt is an open-source community project, it only exists because of those people who have put their time and knowledge into creating it. As it turns out, Ken, one of the authors of the Megasquirt and Spark Extra (MS1 Extra / MSnSE, MS2 Extra, MS3 firmware) firmware, is an avid RX-7 enthusiast. And it is mainly due to his work that we have full rotary support for every current version of the Megasquirt, from MS1 to MS3. Ken, posting on the RX-7 forum as muythaibxr has created an excellent FAQ for MS1 and another FAQ for MSII. I've helped a little with the MS1 FAQ. However, the one disadvantage about the community approach is that information, even with the FAQs, can be a bit splintered and spread across various parts of the Internet. This is the primary reason I've decided to do this writeup. Nowhere has existed a complete 2nd Gen RX-7 Megasquirt write up until now.

A Word Of Warning

Before we go any further, it is important to reinforce that the Megasquirt can be a very Do It Yourself ECU. This means that if you want, you build the ECU. You may have to modify it to work with the rotary, and like any other standalone, you are in charge of wiring it up to make it work in your car.

If you have never soldered before or have no experience in basic electronics, then building a Megasquirt from scratch may not be for you. Pre-assembled Megasquirts which require no modifications to work with the rotary are available (we will cover this later) but if you choose MS1 or MS2, they still require modifications to work with the rotary. MS1/MS2 users will need to add a second reluctor conditioner (we'll cover this) and set up the status LEDs as ignition outputs. While not overly complicated if you can read a schematic, being an electronics novice will make things much more difficult. In this case, choosing a Megasquirt version that requires no modification may be the best bet.

Once the Megasquirt is installed, it must be tuned. A significant stumbling block for first time tuners is simply getting the vehicle to start. With no frame of reference, it is difficult to guess how much fuel the engine needs when starting with a blank map. And I don't think I need to mention how disastrous it can be to tune a rotary too lean. While the maps and fuel/ignition tables I provide will go a long way to guarantee you a successful first startup, they are just a starting point and the responsibility of tuning the car lies with you.

Finally, the Megasquirt is not legal for road use on pollution controlled vehicles. Standalones are for off road use only. While there is no reason at all a Megasquirt controlled vehicle could not pass emissions, most jurisdictions require all factory emissions equipment to be maintained. It's your responsibility to remain compliant with your local emissions laws.

Types Of Megasquirt

Part of the confusion about the Megasquirt world is because the Megasquirt is not a single ECU, it is a collection of them all based upon a common ground. Megasquirts are typically referred to by both their board and processor version. There are 6 major versions (and more minor/specialty versions!) of the Megasquirt main board. Version 1.01 is the original 'Squirt and no longer available. It's highly unlikely you will ever see one. The version 2.2 has some improvements over V1.01 but is no longer being produced in any great quantity and are obsolete. They are still available from the vendors and can be modified for rotary use, but the version 3 is a much better choice. The V3 is a vastly improved design over the V2.2. It is designed to run the MSnSE firmware and offers the option of multiple ignition inputs, an injector flyback circuit and a host of other improvements to the circuit, board layout and board structure. Then there is the V3.57. The V3.57 is essentially the same as the V3, but uses surface mount components and has a few other minor improvements (such as a built in high current idle driver, jumper selectable LED pullups). The Microsquirt is a tiny integrated surface mount board based on the MSII processor and sealed into a weather tight enclosure (also available as a bare board). Finally, there is the MS3-Pro, which is a fully assembled ready to install ECU based on the MS3. Like the Microsquirt, it is also available as a bare board.

Now, all of the boards with the exception of the Microsquirt and MS3-Pro have a choice of three processors. The MS1 is the original CPU and is a 68HC908. The MSII consists of a much faster MC9S12C64 processor mounted on a daughter card. The newest is the MS3, using a MC9S12XEP100 CPU mounted on a daughter card featuring full sequential injection, SD card logging, USB and many more features. MS3 can optionally become MS3X with the addition of another daughter card which supplies the circuitry needed to actually use many of the MS3 features.

To complicate things further, there are many different firmware versions available for different purposes. MS1 chips have one set of firmwares, MSII chips have another, MS3 yet another firmware version with a variant for MS3-Pro ECUs.

Because of all these differences and possible combinations, we will only deal with four configurations in this writeup: the MS1 CPU on the V3 board, the MSII CPU on the V3 board, MS3X (MS3, V3.57 board, MS3X daughter card), and MS3-Pro. MSI will run the MSnSE (MS1/Extra) firmware, MSII will run the MS2/Extra firmware. There is really only one firmware option for the MS3.

Configurations Covered In This Writeup

Because there are so many possible configurations of the Megasquirt, this document will only deal with one specific way of installing each covered version of the 'Squirt. It would be difficult, if not impossible, to account for all scenarios. So we will cover the following setup:

MegaSquirt 1 / MegaSquirt II

Megasquirt I/II Using MSnSE/MSII Extra

V3 Board

Stock CAS with 2nd VR Circuit Added to 'Squirt

Stock 2nd Gen Ignition Coils

AEM UEGO Wideband

GM Air and Coolant Temp Sensors

S4/S5 Stock TPS

Stock BAC Valve w/PWM IAC Mod on 'Squirt

Electric Fan Control w/Relay Control Mod

'Squirt mounted in place of stock ECU with aux fuse panel

MegaSquirt 3 (and MS3-Pro)

Megasquirt 3 w/ MS3X

V3.57 Board

Stock CAS using VR Conditioner On MS3X Board

Stock 2nd Gen Ignition Coils

AEM UEGO Wideband

GM Air and Coolant Temp Sensors

S4/S5 Stock TPS

Stock BAC Valve using Idle Controller On MS3X Board

Electric Fan Control using Spare Injector Output

'Squirt mounted in place of stock ECU with aux fuse panel

Choosing Your MegaSquirt Version and Necessary Modifications To The MegaSquirt

After seeing all those options, which one should you choose?

MegaSquirt I

MegaSquirt 1 is the simplest and oldest version of the MegaSquirt. It will control a rotary when loaded with the MSnSE firmware (as in this writeup). It is only a basic ECU offering a limited feature set including batch fired fuel, spark, closed loop, idle control, a few configurable outputs, staged injection, basic boost control and a few others. Fuel and spark tables are 12 x 12. This makes it easy to tune, and though it will run a car well, I can only recommend it when the user is limited by budget (it is by far the cheapest) or building a very simplistic vehicle (such as a track car). Modifications to the board are needed to run the rotary as well as enable most of the I/O features. In this writeup, these include: 2nd VR circuit, relay control mod, soldering jumpers for VR trigger input, pullup resistors added to status LEDs for ignition outputs. MS1 code development has stopped.

MegaSquirt II

As MegaSquirt 2 uses the same board as MS1, the same hardware modifications are required to control the rotary and enable most I/O. MS2 adds a number of features (too many to list) on top of MS1 but the key improvements are: 16 x 16 tables, closed loop AFR tables (MS1 has this but it is very limited), enhanced acceleration enrichment, usable boost control, and many others. The faster processor allows better accuracy in fuel delivery and ignition timing. Additionally, MS2 code is still actively being developed and some features from MS3 are being back-ported. I would consider that MSII be the minimum MegaSquirt version used on a street car. The cost is approximately 30% higher than MS1. Note MS1 installs can be upgraded to MS2 by simply installing an MS2 CPU daughter card, making a few wiring changes internally, and then retuning the car.

MS3 / MS3-Pro

MegaSquirt 3 w/Expansion board (we will simply call it MS3X from now on) is the newest version of the MegaSquirt and where most of the active development is. The MS3X feature set includes all the functionality of the MS2 (fuel/spark tables are still 16 x 16) but adds a host of new features. The most important of these are full sequential fuel injection. This allows much greater fuel control than the previous batch fired 'Squirts including individual injector trim. Many of the features have been re-worked and improved upon. Large improvements to closed loop boost control, closed loop AFR control, idle control, fan/AC control, engine protection, enhanced acceleration enrichment, idle VE/timing tables. Hardware required for these features is included on the Expansion board meaning that no hardware mods are required to run the rotary and enable most of the I/O when paired with the V3.57 board (as in this writeup). I recommend MS3X for all "street" and "daily driver" vehicles.

The MS3-Pro is a customized version of the MS3 sold by DIYAutoTune. It is a custom board running the MS3 processor. DIYAutoTune has integrated the optional MS3X features such as knock sensing and the real time clock onto their board, as well as using the vastly improved Maxell MAX9926 VR conditioner (a circuit similar to the Microsquirt) at the trigger inputs. Additionally the MS3-Pro uses a compact plastic weatherproof case w/weatherproof Ampseal connectors on the harness. It of course supports all the standard MS3X features and uses the same firmware and tuning software. The MS3-Pro is a good choice for those looking for a "finished, professional" ECU similar to what one would buy from Haltech, for example. Or for the installer who wants a ready to install package.

One important thing to note is that the MS3/MS3-Pro injector drivers are high impedance only. If your car is low impedance you need to either run series resistors (covered later here) or convert to high impedance.

We'll cover this in detail a little later on, but to elaborate a little on the modifications which must be done on MS1/MS2 in order to run the ignition system on the 2nd gen RX-7. First, a second variable reluctor (VR) circuit must be built to allow use of the stock CAS. Second, pullup resistors must be added to the three status LEDs to drive the coils. In addition we will be adding a simple circuit to control an electric fan, and we will also apply the PWM IAC mod to control the RX-7s BAC idle valve.

Getting Down To Business

The rest of this writeup is split up into sections to make reading a bit easier.